1. Field of the Invention
The present invention relates to a method of selectively exciting molecules of a specific species by means of an electromagnetic wave, a method of separating an isotope by using this method and a method of analyzing an isotope. This invention also relates to a selective molecular excitation apparatus and an isotope separation apparatus.
2. Description of the Related Art
Different isotopes of an element have substantially same chemical properties but show a slight difference of mass. Therefore, a specific isotope can hardly be separated from them by way of an ordinary chemical separation method and a special technique is required to separate it. As such a technique, it is known to selectively excite molecules of a specific species including a specific isotope by conducting a particular treatment on a sample and utilize the difference of state in the treated molecules. For instance, ionized atoms are separated from the excited molecules by irradiating the treated sample with an ionizing laser beam so that they can be separated by an electric method with ease. Then, a technique of selectively exciting molecules of a specific species out of a mixture of molecules of various different species takes a vital role.
There is such a known technique of utilizing the energy difference in a state of molecular vibration (an isotope shift) attributable to a slight difference of mass among molecules including different isotopes. With the technique, only molecules of a specific species including isotopes are selectively excited by irradiating a sample with a laser beam having a spectral bandwidth that is sufficiently narrower than the isotope shift. Such selective excitation is difficult when the absorption spectrum of the molecules tends to broaden. Patent Document 1 describes a technique of suppressing the broadening of an absorption spectrum by cooling molecules to a very low temperature.
Similarly, various techniques of efficiently conducting an operation of selectively exciting molecules of a specific species including isotopes by utilizing an isotope shift have been proposed. For example, Patent Document 2 describes a technique of raising the excitation efficiency by sweeping the wavelength of a laser beam. Patent Document 3 describes a technique of raising the excitation efficiency by using a plurality of laser beams of different wavelengths, adjusting the absorption timings of the light beams and utilizing multi-photon absorption.
[Citation List]
[Patent Document]
    [Patent Document 1] Jpn. Pat. Appln. Publication No. 2000-180241    [Patent Document 2] Jpn. Pat. Appln. Publication No. H06-134262    [Patent Document 3] Jpn. Pat. Appln. Publication No. H07-024262
While the technique described in Patent Document 1 can raise the efficiency of selective excitation by suppressing the broadening of an absorption spectrum, it requires a complex treatment process to make it difficult to actually improve the treatment speed because the sample needs to be cooled to a very low temperature. On the other hand, the techniques of Patent Documents 2 and 3 cannot avoid lowering of the efficiency of selective excitation due to a broadening in absorption spectrum.
Furthermore, the rotational quantum number J that defines the actual state of rotation of molecules in a substance varies broadly. Generally, molecules are thermally distributed and hence J of the molecules in a sample shows a certain distribution. Transitions of J take place only in the range of ΔJ=±1 and the energy required for the transition varies as a function of J. When a monochromatic light beam is employed, it can excite only molecules having a specific J. Particularly, when the mass of a molecule (atom) is large, the rotation and vibration frequency of the molecule is small and the density of energy levels thereof rises. For example, rotational quantum numbers not less than 500 and vibrational quantum numbers not less than 20 are significantly observed in molecules of cesium iodide (CsI) at temperature of 1,000K and hence are distributed. Therefore, when a monochromatic light beam is employed, only a single quantum state can be excited out of about 500×20=10,000 quantum states. In other words, the efficiency of excitation is very low. Otherwise, it is only possible to look into the states of molecules (the distribution of J) in molecules of the substance to be treated in advance and selecting a wavelength for a laser beam accordingly.
Thus, it has been difficult to selectively excite molecules of a specific species among molecules of a plurality of species that show only a slight difference of mass.